Twenty years ago, Harvard Business School economist and strategy professor Michael Porter stood conventional wisdom about the impact of environmental regulation on business on its head by declaring that well-designed regulation could actually enhance competitiveness. The traditional view of environmental regulation held by virtually all economists until that time was that requiring firms to reduce an externality like pollution necessarily restricted their options and thus by definition reduced their profits. After all, if profitable opportunities existed to reduce pollution, profit-maximizing firms wouldalready be taking advantage of those opportunities. Over the past 20 years, much has been written about what has since become known simply as the Porter Hypothesis (PH). Yet even today, we find conflicting evidence and alternative theories that might explain the PH, and oftentimes a misunderstanding of what the PH does and does not say. This paper provides an overview of the key theoretical and empirical insights into the PH to date, draws policy implications from these insights, and sketches out major research themes going forward.